Annually, there are approximately 300,000 fusion surgeries performed in the United States. Numerous types of spinal fusion cages exist, varying in design, material, size, and implantation method. Regardless of the type of cage used, a full or partial discectomy is performed prior to implantation. Once the necessary portion of a spinal disc is removed, the disc space is then expanded using a distracter. In order to accept a cage, the disc space must be distracted so that the intervertebral height can be reestablished. Distraction also enhances stability by tensioning the ligamentous apparatus, which increases the compressive forces that hold the cage in place. The amount of disc space distraction is a crucial aspect of the spinal fusion surgery; too little or too much distraction results in various complications that compromise the clinical outcome of the surgery.
All of the current fusion cages can be classified as either threaded or non-threaded fusion cages. Threaded fusion cages are typically cylindrical in shape and are implanted by screwing the cage between the adjacent vertebrae to reestablish the disc space height. Before implantation, the surgeon prepares the vertebral endplates with a reamer, creating a channel for the cage, and then threads the channel using a threading device. By creating the channel parasagitally across the disc space, the vascular cancellous bone is exposed, creating an optimal bleeding bed to promote fusion. However, the strong subchondral bone of the cortical endplate is partially removed, which compromises the endplate's integrity.
Non-threaded cages are typically either box (e.g., rectangular) in shape or cylindrical in shape, and are implanted by impacting the cage into the disc space, reestablishing the intervertebral height. Before implantation, the endplate cartilage is removed in order to expose the bleeding bone. The cage is then inserted into the disc space, usually anchored by saw teeth or spikes, securing it between the adjacent vertebrae. Since only the endplate cartilage is removed, the subchondral bone is preserved, leaving the strongest bone adjacent to the cage. The disc space can also be filled with a greater quantity of bone graft (if used) when compared to the threaded cage, as the cage itself takes up less of the disc space volume. This increased bone graft volume increases the fusion rate. However, the endplate is minimally vascularized, which may delay or impede fusion. The cage must also be precisely the correct height to match the disc space in order for implantation to occur; a factor that makes the implantation of these cages significantly difficult.
When performing an interbody fusion, an anterior or posterior approach is generally used. There is no overall preference as to which surgical approach is to be used, since it is based on the spinal anatomy and the patient's prior surgical history. Anterior lumbar interbody fusion (ALIF) involves accessing the spine through the abdomen. A posterior lumbar interbody fusion (PLIF) procedure gains access to the disc space through the back, avoiding potential complications related to major vascular structures and sympathetic injury. Both the threaded and non-threaded cage varieties can be implanted using either approach.
When implanting a fixed cage, an extensive preparation of vertebral endplates is required in order to properly fit the cage into the disc space. This extensive site preparation can damage the endplates, compromising the integrity of the vertebrae. Since the cage is a fixed height, the surgeon must predetermine the size of the implant. If the cage is too large or too small, the surgeon is forced to coerce the implant to fit into the disc space, risking a malpositioned cage and revision surgery.
An interbody fusion procedure is associated with a 5-10% risk of complication. The vast majority of these complications arise from the surgical procedure used to implant the cage; there are few reported cases of the cage itself failing mechanically. The more common, minor complications resulting from the fusion procedure include dural tears, ileus, superficial infections, and neurologic problems. Other, more serious complications include subsidence, nonunion, device migration, and malpositioning. These complications usually result in a revision surgery, in which the surgeon must perform a second procedure to remove the initial cage, repair any damage, and implant a new device.
Subsidence occurs when the implant penetrates the vertebral endplate. This penetration itself can cause pain and lead to a loss of the disc space height that was originally achieved. The loss of disk space height negates the implant and the spine is once again unstable. This instability leads to revisions in an attempt to alleviate the pain and re-space the vertebrae again. Subsidence also causes a narrowing of intervertebral foramen and loss of lordsosis, which further exacerbates patient discomfort and can impair the patient's balance. The problem of subsidence is one of the most prominent in spinal fusions. However, it has been found that having a footprint surface area of the implant greater than 40% the surface area of the vertebral endplate greatly reduces the incidence of subsidence.
Nonunion, or pseudarthrosis, is a major concern in interbody fusion as it negates the desired effect. Pseudarthrosis can occur for several reasons, the most common involving motion about the cage. An improperly sized implant may fail to gain adequate purchase into the bony endplates, leading to laxity and nonunion. In order for a proper fusion to take place, the surrounding bone must be subjected to sufficient loads so that bone formation is generated. Further, micromotions of the implant and implant-bone interface must be kept to a minimum to promote bone ingrowth. At micromotions above 24 μm, fibrous tissue ingrowth begins to occur, which impedes bone ingrowth and creates an improper fusion. If pseudarthrosis occurs, a revision surgery is performed so that the segment can be properly stabilized.
In extreme cases of nonunion, device migration can occur. A cage that is not securely fixed to the adjacent vertebrae can migrate anteriorly or posteriorly, both of which have drastic effects. Anterior migration results in the cage moving into the abdomen. During motion, the migrating implant can tear major vascular structures and organs, causing extensive complications and even death. If the cage migrates posteriorly, it can shift into the medullary canal, damaging the spinal cord, causing paralysis and death. An implant migration requires immediate revision surgery. These revisions are especially taxing, as the implant must be located and extracted and any extensive damage it may have caused must be repaired.
A malpositioned cage can be the underlying cause for a variety of complications, including the three previously discussed. A cage that is malpositioned is one that either does not appropriately fit the disc space or was implanted in an incorrect manner. Malpositioning can be the result of a poorly designed cage, an inappropriate device selection, or surgeon error. The faulty placement of the cage can result in decreased stability, which can cause pseudarthrosis or migration. A cage that is inappropriately sized can increase the risk of subsidence or compress surrounding nerve roots. Since the positioning of the cage is causing substantial complications, a malpositioned cage must be removed and replaced via a revision surgery.
These four complications account for the vast majority of revision surgeries performed. Each year in the United States, approximately 6% of fusions result in revision surgery. Other complications, such as vascular and neurological injury are more frequent, but normally do not require a revision surgery. The main vascular injuries that were found included arterial thrombosis, venous thrombosis, and lacerations. The complications resulting in a revision surgery are mostly due to the surgical procedure involved in interbody fusion.
The endplate preparation required to implant the cage also causes endplate abrasion and is further exacerbated during the implantation of the cage, in which the surgeon must coerce the implant into the disc space. This situation is particularly common with non-threaded cages. The surgeon distracts the disc space to the same height as the cage, therefore requiring substantial force to impact the cage into the void. The shear stresses against the vertebrae erode the endplates, damaging their surfaces. This trauma to the endplates initiates an inflammatory response, which can delay or inhibit fusion. The lack of stability of the implant can then result in pseudarthrosis or migration, requiring a revision surgery.
During a fusion surgery, the intricate procedure required to implant the cage often leads to surgeon error. A more common surgeon error is an inaccurate distraction of the intervertebral space. The amount of distraction is one of the most important aspects of a fusion surgery. Over-expansion can stretch and irritate the surrounding muscular and neural tissue, while too little expansion will not allow for adequate tensioning of the ligamentous apparatus, decreasing the stability of the cage. Subsidence, pseudarthorsis, or possibly migration could result, necessitating a revision surgery.
X-rays are generally used to assess the disc space height in order to select a cage of the appropriate size. Height will be measured again during surgery, but once the cage is implanted, the height cannot be adjusted. The height of the implant is one of the most important aspects of the fusion surgery. Over-distraction from implants that are too tall will stretch the surrounding neural and muscular tissue, causing irritation. The widened disc space will also compress the adjacent discs, placing them under abnormal stresses and depleting their shock absorbing function. The large implant will be more likely to subside into the surrounding vertebrae, as the disc space will attempt to return to its normal height. An implant that is too small will fail to gain sufficient purchase into the vertebral endplates, resulting in pseudarthrosis and possibly migration. In both cases, the risk of malpositioning is increased, since the implant is inherently poorly suited for the intended space. A revision surgery will most likely be required.
The revision surgery is strenuous for both the surgeon and patient. Revision surgeries are generally longer and more extensive than the original surgery, as the faulty implant must first be removed. This increased surgical time imposes more trauma to the patient, and is usually associated with a significantly increased recovery time. The revision surgery also increases health care costs, requiring the hospital to spend substantial funds.
A significant portion of complications that lead to a revision surgery are caused by a mismatch between the intervertebral height and fusion cage, which is mostly due to the fixed height nature of the current cages. Since the surgeon must determine the height of the cage before implantation, any error in this measurement will result in a mismatch between cage and disc space height. The surgeon is unable to alter the height intraoperatively, and must therefore coerce the cage to fit or, if available, implant a different cage. Even if the cage is sized appropriately, some coercion is needed to impact the cage into the disc space, resulting in endplate abrasion and possible malposition.
To summarize, current fixed cages present the potential problems of abrasion, subsidence, malpositioning, pseudoarthritis, and even cage migration, all of which create potential further future expense and pain for patients and clinicians by means of the need for revision surgeries. What is needed, therefore, is a cage that minimize or eliminate the foregoing problems characterized by fixed cages.